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Date: Sun, 14 Oct 2007 07:21:06 -0400
From: "Peter C. Chapin" <pchapin@sover.net>
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Subject: Re: Too detailed public view of POSIX.IO.File_Descriptor in Florist?
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Date: 2007-10-14T07:21:06-04:00
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Jacob Sparre Andersen wrote:

> Doesn't Florist (the POSIX.5 interface) give us a too detailed view of
> POSIX.IO.File_Descriptor?
> 
> Is there any need at all for knowing that a file descriptor is an
> integer type?

I don't know anything about Florist. However POSIX does guarantee that
open(2) returns "a non-negative integer representing the lowest numbered
unused file descriptor." This means that the actual values of the file
descriptors are significant and can be meaningfully compared.

I/O redirection depends on this. If I have all file descriptors in the
range 0..n open and I close a descriptor i that is less than or equal to
n, I know that the next time I open a file it will be on descriptor i.
Since the standard input is descriptor zero and the standard output is
descriptor one, I can close these descriptors explicitly and then open a
file that then appears to be my standard input or output accordingly.
Thus, in C,

	close(0);
	open("some/file.txt", O_RDONLY);

Now when the program reads its standard input it actually gets
some/file.txt.

Peter